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Prostate cancer progresses so slowly in most cases that more men die with the disease than of it, research shows.

So some patients, concerned about the risk of impotence or incontinence that accompanies aggressive treatment, opt for “active surveillance,” sometimes called “watchful waiting.” Instead of undergoing immediate treatment, these men typically are monitored with blood tests, biopsies and sonograms. They can be treated if their cancer grows or appears to become more aggressive.

But the results of a recently published study of Swedish men might have unnecessarily alarmed prostate cancer patients who’ve opted for active surveillance.

Between 1989 and 1999, researchers randomly assigned 695 patients under age 75 with low-risk prostate cancer that hadn’t spread to the bone to either watchful waiting or a radical prostatectomy, an operation in which the prostate gland and some surrounding tissue are removed. The researchers then followed the patients through the end of 2012.

In early March, they reported in The New England Journal of Medicine that they’d found a “substantial reduction” in the risk of death among men who had the surgery, especially in men younger than 65. In the nearly quarter-century of follow-up, 63 men in the surgery group died of prostate cancer, compared to 99 in the watchful-waiting group.

Even before the Swedish study was published, many candidates for active surveillance — generally older men who have cancer that doesn’t appear to be aggressive or to have spread beyond the prostate — chose to undergo surgery. They couldn’t imagine living with the knowledge that their prostate harbored a malignant tumor.

Nuances of the Research

However, an American Cancer Society official, as well as one of the study’s co-authors, agree that the newly published research results shouldn’t send prostate cancer patients running to the OR.

That’s because the screening and treatment landscape in the United States in 2014 differs significantly from that of Sweden in the 1990s, when most of the study participants were diagnosed.

“There are so many nuances in this paper, and so many potentially misunderstood points that are made,” says Dr. Len Lichtenfeld, deputy chief medical officer for the cancer society. “This is a study that was started in the late 1980s … We have to understand the research as it was done in the context of what we knew at that time.”

For example, Lichtenfeld says, nearly nine in 10 of the prostate cancers diagnosed in the Swedish trial were detected because they were already big enough for doctors to feel them. Only a small minority were picked up with a PSA test, which measures blood levels of prostate specific antigen, a protein produced by the prostate gland.

Today, though, the vast majority of prostate cancers are diagnosed because of a PSA test, which can suggest the presence of tumors a decade before they might be large enough to be felt on examination, says Jennifer Rider, a co-author of the Swedish study and an epidemiologist at the Harvard School of Public Health.

Many of these early, slow-growing tumors are unlikely to cause any symptoms, and autopsy studies have found that a large percentage of men who die of other causes are found to have undiagnosed prostate cancer that never bothered them.

“The other big difference is that our comparison group was watchful waiting,” Rider says. Although the term is sometimes used interchangeably with “active surveillance,” they’re not the same, at least in her eyes and those of her co-authors, she says.

Watchful Waiting Vs. Active Surveillance

For one, men in her study’s watchful waiting group received treatment only for symptoms, not with the goal of curing their cancer if it appeared to have grown or become more aggressive. “Men who are under active surveillance protocols today would be more likely to receive interventions,” Rider says.

Four out of 10 men in the study’s watchful waiting group never required any treatment for symptoms, she says. “There are still a large percentage of men who could really live after their prostate cancer diagnosis and be left alone, and that’s an important thing to keep in mind,” Rider adds.

In addition, the men in the surgery group were nearly four times as likely to report urine leakage as those in the watchful waiting group, or 41 percent vs. 11 percent. About four out of five men in each group reported erectile dysfunction.

“Distress from these symptoms was reported as being significantly greater in men assigned to radical prostatectomy than watchful waiting,” Rider and her co-authors write.

Although such long-term studies don’t use current state-of-the-art care, Lichtenfeld says, “they’re important to inform our knowledge about how the disease behaves.”

The most important thing is that men diagnosed with prostate cancer understand their options, he says. “They need to understand they have choices about being treated or not being treated," says Lichtenfeld. "We have options that are now gaining traction that offer more men the opportunity not to have aggressive therapy.”

Rita Rubin is a former USA Today medical writer who now writes about health and science for publications including Next Avenue, WebMD and NBCNews.com.